A technique that is extensively used for producing coatings upon a wide variety of substrates entails the exposure of the workpiece (normally grounded) to a cloud of electrostatically charged particles, thus producing a deposit that can subsequently be integrated. Although there is now a substantial body of prior art, fluidized bed apparatus that is especially effective for use in such applications is disclosed in Goodridge U.S. Pat. No. 3,828,729, issued Aug. 13, 1974, Knudsen U.S. Pat. No. 3,916,826, issued Nov. 4, 1975, and Karr U.S. Pat. No. 4,030,446, issued Jun. 21, 1977. In Hajek U.S. Pat. No. 4,848,432, issued Feb. 28, 1989, an electrostatic powder coating unit is provided in which a tubular cloud of charged particles is induced to move helically within a cylindrical porous member, to coat workpieces conveyed through it.
Systems of the kind described have been employed with the workpieces moving in a variety of different ways; most commonly, however, the workpiece will be conveyed past the coating unit along either a horizontal or a vertical path. Illustrative vertical systems are disclosed in Smyser U.S. Pat. No. 22,419, issued Jan. 11, 1944, and Hug U.S. Pat. No. 2,706,963, issued Apr. 26, 1955 (note FIG. 3). Furthermore, both horizontal and vertical electrostatic fluidized bed coating systems are commercially available.
Despite the notable advances that have occurred in the art, as indicated by the foregoing, increased coating efficiencies and enhanced performance are normally considered to be highly desirable achievements. Accordingly, it is the broad object of the present invention to provide a novel apparatus, system and method for effecting the coating of a workpiece from an electrostatic fluidized bed, by which are afforded substantially increased levels of efficiency of deposition of the particulate material, and improved performance.
A more specific object of the invention is to provide such apparatus which is especially suited for operation in a vertical mode, and to provide such a system and method in which the workpiece is transported along a vertical path during coating.
Other specific objects are to provide an apparatus, system and method having the foregoing features and advantages, which are particularly well adapted for coating of continuous length, web-like substrates.
Further objects of the invention are to provide such an apparatus and system which are of relatively incomplex and inexpensive design and construction, and to provide such a method which is facile and convenient to carry out.